Optical platform to enable efficient LED emission

ABSTRACT

An integrated multi-layer apparatus and method of producing the same is disclosed. The apparatus comprises an LED, a beam shaping layer, and a refracting layer between the beam shaping layer from the LED. The refracting layer may have an index of refraction lower than the index of refraction of the LED and the beam shaping layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/201,936, filed Aug. 29, 2008 now U.S. Pat. No. 7,839,062, thecontents of which are hereby incorporated by reference herein in theirentirety.

FIELD

The present disclosure relates generally to the field of semiconductorlight emitting devices, and more specifically, to recycling optics forlight emitting diodes.

BACKGROUND

A light emitting diode (LED) is a semiconductor material impregnated, ordoped, with impurities. These impurities add “electrons” and “holes” tothe semiconductor, which can move in the material relatively freely.Depending on the kind of impurity, a doped region of the semiconductorcan have predominantly electrons or holes, and is referred as n-type orp-type semiconductor regions, respectively. In LED applications, thesemiconductor includes an n-type region and a p-type region. A reverseelectric field is created at the junction between the two regions, whichcause the electrons and holes to move away from the junction to form an“active region.” When a forward voltage sufficient to overcome thereverse electric field is applied across the p-n junction, electrons andholes are forced into the active region and combine. When an electroncombines with a hole, it falls to a lower energy level and releasesenergy in the form of light.

The light emission profile from an LED is non-directional and typicallyassumes a “lambertian” like profile where light is equally emitted intoall directions. However, in many applications, only focused light isuseful. Typically, secondary optics are required to capture and shape alarger portion of the total available light from the LED. Thesesecondary optics are costly and increase the size of the overall packagecarrying the LED.

Accordingly, it would be desirable to shape the emission profile from anLED such that the emitted light can more readily be used withoutexternal complicated and costly secondary optics.

SUMMARY

In one aspect of the disclosure, an integrated multi-layer apparatusincludes an LED configured to emit light having a first portion emittedwithin an angular range and a second portion emitted outside the angularrange; a beam shaping layer configured to pass the first portion of theemitted light and reflect the second portion of the emitted light backto the LED; a refracting layer between the beam shaping layer and theLED, the refracting layer having an index of refraction lower than theindex of refraction of the LED and the beam shaping layer; wherein theLED is further configured to randomly scatter the second portion of theemitted light reflected by the beam shaping layer and redirect thescattered light back to the beam shaping layer.

In another aspect of the disclosure, an integrated multi-layer apparatusincludes an LED configured to emit light having a first portion emittedwithin an angular range and a second portion emitted outside the angularrange; a beam shaping layer configured to pass the first portion of theemitted light and reflect the second portion of the emitted light backto the LED; a refracting layer between the beam shaping layer and theLED, the refracting layer having an index of refraction lower than theindex of refraction of the LED and the beam shaping layer; wherein theLED further comprises a roughened surface in communication with therefracting layer and a back reflector.

In yet another aspect of the disclosure, an integrated multi-layerapparatus includes light emitting means for emitting light, wherein thelight includes a first portion emitted within an angular range and asecond portion emitted outside the angular range; beam shaping means forpassing the first portion of the emitted light and reflecting the secondportion of the emitted light back to the light emitting means;refracting means for refracting the emitted light, the refracting meanshaving an index of refraction lower than the index of refraction of thelight emitting means and the beam shaping means; wherein the lightemitting means comprises means for randomly scattering the secondportion of the emitted light reflected by the beam shaping means andmeans for redirecting the scattered light back to the beam shapingmeans.

In a further aspect of the disclosure, a method of emitting light froman integrated multi-layer structure is described. The integratedmulti-layer structure includes an LED, a beam shaping layer, and arefracting layer between the LED and the beam shaping layer, wherein therefracting layer comprises an index of refraction lower than the indexof refraction of the LED and the beam shaping layer. The method includesemitting light from the LED, wherein the light includes a first portionemitted within an angular range and a second portion emitted outside theangular range; passing the first portion of the emitted light throughthe beam shaping layer and reflecting the second portion of the emittedlight from the beam shaping layer back to the LED; randomly scattering,at the LED, the second portion of the emitted light reflected by thebeam shaping layer; and redirecting the scattered light from the LEDback to the beam shaping layer.

It is understood that other aspects of the present disclosure willbecome readily apparent to those skilled in the art from the followingdetailed description, wherein it is shown and described only exemplaryaspects of the disclosure by way of illustration. As will be realized,the disclosure includes other and different aspects and its severaldetails are capable of modification in various other respects, allwithout departing from the spirit and scope of the present disclosure.Accordingly, the drawings and the detailed description are to beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present disclosure are illustrated by way ofexample, and not by way of limitation, in the accompanying drawings,wherein:

FIG. 1 is a cross-section view illustrating an example of an LED; and

FIG. 2 is a cross-section view illustrating an example of an integratedmulti-layer apparatus.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various aspects of the presentdisclosure and is not intended to represent all ways in which thepresent disclosure may be practiced. The detailed description mayinclude specific details for the purpose of providing a thoroughunderstanding of the present disclosure; however, it will be apparent tothose skilled in the art that the various aspects of the presentdisclosure may be practiced without these specific details. In someinstances, well-known structures and components are summarily describedand/or shown in block diagram form in order to avoid obscuring theconcepts of the present disclosure.

In several configurations of an apparatus, the emission profile isshaped such that the emitted light can more readily be used withoutexternal complicated and costly external secondary optics. Furthermorebecause no secondary optical devices are used, as described below, theresulting size of the overall package is small. The brightness ofemitted light may be improved due to the methods of light recyclingemployed. Various aspects of the disclosure are based on geometricaloptics and thus unlike diffractive or photonic crystal structures, aremuch less sensitive to polarization and wavelength and can be tailoredfor various applications.

Various aspects of this disclosure will be described in terms of anintegrated multi-layer apparatus. As used herein, an integratedmulti-layer apparatus is intended to cover a structure having multiplelayers. Each layer of the apparatus may itself comprise several layersor sub-layers. By way of example, an integrated multi-layer apparatusmay include an LED layer having an active region sandwiched between twooppositely doped epitaxial layers all of which are formed on a growthsubstrate. A beam shaping layer formed on the LED may include severallayers of material deposited onto a substrate such as glass. In otherwords, the term “layer” used throughout this disclose does notnecessarily denote a homogeneous layer of material.

As those skilled in the art will readily appreciate, when a layer isreferred to as being “on” another layer, it can be directly on the otherlayer or intervening layers may be present. For instance, the precedingreference to a beam shaping layer on an LED does not precludeintervening layers between the two. In a configuration of a multi-layerapparatus discussed below, a refracting layer is formed between the LEDand the beam shaping layer.

The “integration” of these layers into the apparatus means that thelayers are formed together by suitable means, now known or laterdiscovered. By way of example, the LED may be grown on a substrate andthe beam shaping layer may be adhered, bonded or otherwise applied tothe LED, either directly or through an intervening layer (e.g.,refracting layer).

Various aspects of an integrated multi-layer apparatus are describedherein with reference to cross-sectional view illustrations that areconceptual in nature. Various layers of the apparatus should not beconstrued as limited to the particular configuration shown in thedrawings. By way of example, the “layers” of an integrated multi-layerapparatus are shown with discrete physical boundaries. However, inpractice, a concentration gradient may exist across the physicalboundaries between layers with the material from one layer penetratingthe material of adjacent layers in either a controlled or randomfashion. Thus, the layers illustrated in the drawings are conceptual innature and their shapes are not intended to illustrate the precise shapeof a layer and are not intended to limit the scope of the invention.

Those skilled in the art will further appreciate that relative termssuch as “top” or “bottom” (and similar terms) may be used herein todescribe a relationship between layers. Notwithstanding the use of suchterms, those skilled in the art will readily understand that theconcepts presented throughout this disclosure are intended to extend todifferent orientations of an integrated multi-layer apparatus inaddition to the orientation depicted in the drawings.

Turning to FIG. 1, an LED with a vertical structure is shown. However,as those skilled in the art will readily appreciate, the various aspectspresented throughout this disclosure are likewise applicable to otherLED configurations, as well as other light emitting semiconductors, nowknown or later discovered. Accordingly, any reference to a verticalstructure LED is intended only to illustrate various aspects of anintegrated multilayer apparatus, with the understanding that suchaspects have a wide range of applications.

FIG. 1 is a cross-section view illustrating an example of an LED. Inthis example, the LED 10 has a vertical current injection configuration,including an n-type contact (or n-type electrode) 11, an n-typesemiconductor layer 12, an active region 13, a p-type semiconductorlayer 14, a p-type contact (or p-type electrode) 15, a thermally and/orelectrically conductive substrate 16 to support the LED structurally,and a back reflector 17. The LED may be fabricated using known processeswith a suitable process being a fabrication process using chemical vapordeposition. The LED may be formed on a wafer and then singulated formounting in a package. The growth substrate may remain as part of thesingulated LED or the growth substrate may be fully or partiallyremoved.

As the n-type semiconductor layer 12 and the p-type semiconductor layer104 are opposite to each other, together they form a pair of carrierinjectors relative to the active region 13. Therefore, when a voltage isapplied to the LED 10, electrons and holes will be combined in theactive region 13, thereby releasing energy in the form of light. If anincident angle of light at the interface between the n-typesemiconductor layer 12 and the ambient air (or other encapsulatingmaterial) is greater than a critical angle in accordance with Snell'slaw, a portion of light generated inside the LED 10 device may gettrapped inside the LED 10 due to total-internal-reflection (TIR). Toincrease the chance of light escaping from the LED, the n-typesemiconductor layer 12 is roughened. Alternatively, or in addition to,the bottom of the p-type semiconductor layer 16 may be roughened (notshown). The roughened surface scatters the normal incident light inrandom directions and reduces the effects of TIR. The back reflector 17may be provided at the bottom of the LED 10 for redirecting lightemitted from the active region 13 back toward the top surface of the LED10. Alternatively, or in addition to, one or more the sides of the LED10 may also have a back reflector.

In one configuration, as shown in FIG. 2, an integrated multi-layerapparatus 99 includes a beam shaping layer 100; a refracting layer 102;and an LED 104. The LED 104 emits light through the refracting layer 102to the beam shaping layer 100. The beam shaping layer 100 may be anoptical element or filter that passes an angular range of incident lightand reflects light falling outside the angular range due to TIR. Theoptical element may be formed by depositing several layers of materialonto a substrate (e.g., glass) by a physical vapor deposition processsuch as evaporative or sputter deposition or a chemical process such aschemical vapor deposition. An example of an optical element is aperiodic prism structure manufactured by 3M under the trademark Vikuiti™Brightness Enhancement Film (BEF). Other optical elements that may beused include periodic structures formed with a number of lenses,mirrors, prisms, or other optical components, or any combinationthereof. In this example, the light falling outside the angular range isreflected by the beam shaping layer 100 back to the LED 104. Thereflected light is then randomly scattered by the roughened surface ofthe LED and redirected back to the beam shaping layer 100 by the backreflector (see FIG. 1).

The refracting layer 102 is a layer which has an index of refractionwhich is less than the index of refraction of the beam shaping layer 100and the LED 104. The refracting layer 102 may be a material such as airor some other suitable material. In the case of air, the multilayerapparatus 99 may be physically constructed to provide an air gap betweenthe LED 104 and the beam shaping layer 100 by means well known in theart. In this configuration, the refracting layer 102 separates the beamshaping layer 100 from the LED by at least 1 λ/nL to ensure no directcoupling between the two takes place, where λ is the wavelength, and nLis the index of refraction of the refracting layer 102.

In the integrated multi-layer apparatus 99 of FIG. 2, light generated bythe LED 104 gets refracted as it travels through the low-index region ofthe refracting layer 102 to the high-index region of the beam shapinglayer 100, thereby causing high angle light propagating through therefracting layer 102 to bend to a lower angle within the beam shapinglayer 100. This enables narrower angle light to be incident onto thebeam shaping layer 100, thereby reducing the amount of light that isreflected back to the LED 104. As a result, the emission profile shouldbe narrower than without the refracting layer.

In a configuration, it is possible to employ a one-dimensional structurein the beam shaping layer 100. This would be useful in applications suchas side view lighting for LCD display. In other applications, such asdirect view backlight for liquid crystal display (LCD) TVs, most of thelight is kept at a high angle. Yet, in other applications, thedivergence is narrowed in one direction and is increased in orthogonaldirections. All these different types of emissions can be obtainedthrough the beam shaping layer 100 of the configuration shown in FIG. 2.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but are to be accorded the full scope consistentwith the language of the claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. All structural andfunctional equivalents to the elements of the various aspects describedthroughout this disclosure that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the claims. Moreover,nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims. No claim element is to be construed under the provisions of 35U.S.C. §112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

1. A method of emitting light from an integrated multi-layer structurehaving a LED, a beam shaping layer, and a refracting layer between theLED and the beam shaping layer, wherein the refracting layer comprisesan index of refraction lower than the index of refraction of the LED andthe beam shaping layer, the method comprising: emitting light from theLED, wherein the light includes a first portion emitted within anangular range and a second portion emitted outside the angular range;passing the first portion of the emitted light through the beam shapinglayer and reflecting the second portion of the emitted light from thebeam shaping layer back to the LED; randomly scattering, at the LED, thesecond portion of the emitted light reflected by the beam shaping layer;and redirecting the scattered light from the LED back to the beamshaping layer.
 2. The method of claim 1 wherein the beam shaping layercomprises periodic optical structures.
 3. The method of claim 1 whereinthe beam shaping layer comprises at least one of a lens, mirror, orprism.
 4. The method of claim 1 wherein the beam shaping layer and theLED are arranged to avoid any direct couplings between the two.
 5. Themethod of claim 1 wherein the refracting layer separates the beamshaping layer from the LED by at least 1 λ/nL.
 6. The method of claim 1wherein the refracting layer comprises air.
 7. The method of claim 1wherein the LED comprises a roughened surface, and wherein the secondportion of the emitted light is randomly scattered by the roughenedsurface of the LED.
 8. The method of claim 1, wherein the LED comprisesa back reflector, and wherein the scattered light is redirected back tothe beam shaping layer by the back reflector.